Provided is a method for mixing a mutually soluble first liquid and second liquid in a microchannel mixing flow path. This method involves a step in which the first liquid and the second liquid are merged in the mixing channel, and a step in which an insoluble fluid insoluble with the two liquids to be mixed is supplied, from a direction intersecting the flow path, into the merged liquid flowing through the flow path, separating the merged liquid at intervals; thereby, mixing cells (60) of the merged liquids, and insoluble fluid cells (63) of said insoluble fluid form an alternating slug flow in the flow path downstream from the insoluble fluid supply device, and, by this means, the first liquid to be mixed and the second liquid to be mixed disposed in each mixing cell are mixed in the aforementioned downstream flow path.